1. Field of the Invention
This invention relates to a rotary slide valve for assisted steering systems with a valve bushing placed in a housing and enclosing a rotary slide. These two valve components are arranged rotatably with respect to each other. The valve bushing is connected with a drive shaft and the rotary slide is connected with a steering shaft that carries a manual steering wheel. A rotatable rod is inserted between the valve bushing and the rotary slide. Furthermore, between the valve bushing and the rotary slide, there is provided a centering device that consists of inclined surfaces with centering bodies located therebetween and a spring means useful for precise central positioning of the rotary slide valve. The force of the spring that acts on the centering device can be superposed by an adjustable pressure force that works in a reaction chamber. The pressure force can be adjusted in assisted steering systems, for example, by means of a speed dependent signal. In this way, a small reaction force works on the manual steering wheel when driving slowly, whereas a comparatively strong reaction force is at work when driving fast. By means of this, one can impart good roadway surface contact and thus a good steering sensation, especially at faster speeds.
2. Description of the Prior Art
From a prospectus of the firm of TRW dating back to the year 1990 entitled "Rack and Pinion Steering," pages 10 and 11, a steering system is known that is provided with a rotatable rod connecting the steering shaft and a drive shaft, as well as a centering device that resets a rotary slide valve into the exact middle position. In this kind of assisted steering, the rotary slide is a component of the steering shaft and the valve bushing is a component of the drive shaft. The opposing fronts of a spring-loaded reaction disc that is connected with the steering shaft and of the valve bushing are provided with opposing recesses that form inclined surfaces to receive catch balls. The steering shaft with the reaction disc can be turned with relation to the valve bushing only when the torque, exercised by the driver on the manual steering wheel, is sufficient to overcome the spring prestress. At that point, the rotary slide valve can be deflected in order to select a pressure in the pertinent servomotor. In front of the reaction disc on the side of the catch balls there is a pressure chamber with an outlet duct whose return cross-section is adjustable. By adjusting the oil pressure it is possible to change the spring prestress and thus the torque that is required on the manual steering wheel to activate the auxiliary power. The regulation of the oil pressure behind the reaction disc is accomplished by a magnetic valve that is controlled by an electronic control unit that responds to the speed-dependent signals. The diameter of the reaction disc must be sufficiently large so that--to reduce the comparatively strong spring force--a sufficiently strong hydraulic pressure force can act as a counterforce. This means that the valve housing must be made large in the area of the reaction disc and the spring.